1. Field of the Invention
The present invention relates to a self-cleaning method for an aligner (lithograph apparatus), and an aligner.
2. Description of the Related Art
During pattern formation of wires and elements in a semiconductor integrated circuit structure, photolithograpy technology is employed to perform patterning. Specifically, the pattern is formed by irradiating photoresist coated onto a semiconductor wafer with light using an aligner.
With the recent increase in the degree (density) of integration of semiconductor circuits, demands have been made for finer, higher density pattern formation. In response to these demands, techniques for reducing the wavelength of the exposure light source are progressing. At present, an ArF Exclmer light source having a wavelength of 193 nm, an F2 light source having a wavelength of 157 nm, and soft X rays such as EUV are either in practical use or under development.
With an aligner which performs fine, high-density pattern formation using this type of light source, contaminants in the air adhere to the surface of the optical components, causing adverse effects such as decreased illuminance and uneven illuminance. Therefore, a nitrogen purge is performed to reduce the effect of the contaminants. This purges the optical path with nitrogen gas at all times. However, even when the nitrogen purge is performed sufficiently, adverse effects such as decreased illuminance and uneven illuminance are caused by contamination of the optical components because the optical path system is exposed to the atmosphere during maintenance, and because contamination in fact occurs even during a normal exposure operation.
Hence, an aligner which employs an exposure light source having a particularly short wavelength adopts a self-cleaning method, which is known as optical cleaning. In this self-cleaning method, the optical system is irradiated with ultraviolet rays issued from a light source at a wavelength of no more than 200 nm. The intense energy of the ultraviolet rays severs the chemical bonds of the carbon compounds which are the main component of the contaminants adhered to the surface of the optical system, thereby breaking down the contaminants.
For example, Japanese Patent Application Kokai (Laid-Open) No. 5-335206 discloses a projection aligner including an illumination optical system for outputting illumination light, a filter which transmits only wavelengths that are useful for exposing, an exposure mask on which a desired exposure pattern is formed, a projection optical system for projecting the exposure pattern onto an object to be treated, a holder on which the object to be treated is mounted, and a stage for moving the holder relative to the projection optical system. During self-cleaning, an ultraviolet filter is positioned on the optical path in place of the filter and exposure mask, the holder is irradiated with the ultraviolet rays that pass through the ultraviolet filter via the projection optical system, and thus organic foreign bodies and impurities which have become adhered to the holder are oxidized and removed.
Similarly, Japanese Patent Application Kokai No 10-335235 and Japanese Patent Application Kokai No. 10-335236 disclose an aligner which is constituted such that light diffusing means or an optical path deflector may be disposed in the position of an exposure mask in order to form another optical path which is different from the optical path used during normal exposure.
Japanese Patent Application Kokai No. 2000-91207 discloses an oscillating concave mirror disposed on a stage for reflecting the light from the light source toward an optical system. Japanese Patent Application Kokai No. 2002-164267 discloses a plane mirror disposed on a stage for reflecting the light from the light source toward an optical system.
However, these conventional aligners have the following problems.
In Japanese Patent Application Kokai No. 5-335206, the ultraviolet filter is positioned on the optical path in place of the filter and exposure mask, and hence the ultraviolet rays used for optical cleaning are emitted onto the same location of the same path as those used during normal exposure. As a result, contaminants that are adhered to locations other than the normal exposure path cannot be cleaned away, and these remaining contaminants may move to the exposure optical path and produce adverse effects.
In Japanese Patent Application Kokai Nos. 10-335235, 10-335236, and 2000-91207, not only is the aligner complex, but moreover, existing aligners cannot be used. In Japanese Patent Application Kokai No. 2002-164267, the optical path irradiated with the light reflected by the plane mirror is limited, and hence it is difficult to achieve effective and desired cleaning.